El Tió de Nadal is a traditional Catalan Christmas log, which is covered with a cloth and on Christmas Eve children hit it with a stick and sing a rude song and magically the log poops out Christmas presents.
There are many variants on the tradition, which are celebrated in Aragon, Andorra and Southern France, but here in Catalonia, a trunk is typically brought into the house at the start of Advent.
It is covered with a cloth so that it doesn't get cold and fed every day so that it won't have any trouble going to the toilet.
Then on Christmas Eve, children hit the log with a stick and sing the Caga Tió song and as if by magic, sweets, nuts and fruit come out from under the cloth.
Modern Tiós tend to have a face painted on them and wear a traditional Catalan barretina but even today, many rural families just bring a log in from the local woods and keep to the tradition in the same way it has been celebrated for centuries.
On Christmas Eve or before or after lunch on Christmas Day, children are sent out of the room traditionally to pray or sing carols when they come back, they hit the log in turns with a stick while singing the Caga Tió song.
An adult has a stick under the cloth and as the children sing and hit, sweets, nuts and fruit appear from under the cloth. The game is normally over when something inedible such as a piece of coal, an onion or a raw herring appears or alternatively when the log pees itself. Here's part of what they sing.
Caga tió, tió de Nadal
No caguis arengades, que són salades
Caga torrons, que són més bons!
Shit log, Christmas log
Don't shit herrings, which are salty
Shit torrons, which are nicer!
Traditionally, there no big presents in the Tió de Nadal because the main gifts are brought by the Three Kings on January 6th but rather sweets and goodies that can be eaten on Christmas Day and Boxing Day or Sant Esteve.
It's also typical that other members of the family and friends will have a Tió in their houses so children tend to be quite enthusiastic about making Christmas visits because they have another chance to Fer Cagar El Tió!
It appears that the tradition is associated with nature and fertility and is closely connected with the winter solstice which is a few days before Christmas.
Interestingly, the Caga Tió is not the only Catalan Christmas tradition that involves defecating because there is also El Caganer, the Catalan peasant who can be found doing his necessities in traditional nativity scenes.
If you're visiting Barcelona as a tourist, you can see the Tió de Nadal in action at the Fira de Santa Llúcia Christmas Market throughout the month of December.